1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor containing an amine compound which can effectively prevent image defects due to oxidizing gases such as ozone and NOx, and an image forming apparatus including the photoreceptor.
2. Description of the Related Art
An image forming apparatus of an electrophotography system (hereinafter, also referred to as an “electrophotographic apparatus”), which forms images using an electrophotographic technology, is widely used in copying machines, printers, facsimile machines and the like.
In an electrophotographic apparatus, an image is formed through the following electrophotographic process. First, a photosensitive layer of an electrophotographic photoreceptor (hereinafter, also referred to as a “photoreceptor”) included in the apparatus becomes charged, and then is exposed to form an electrostatic latent image. The formed electrostatic latent image is developed to form a toner image, and the formed toner image is transferred onto a transfer material such as recording paper and fixed to form a desired image on the transfer material.
In recent years, the electrophotographic technology is used not only in the field of a copying machine but also in the field of a printing plate material, a slide film, and a microfilm, in which a silver-salt photographic technology has been conventionally used. For example, the electrophotographic technology is applied to a high-speed printer which uses laser, a light emitting diode (LED), or a cathode ray tube (CRT) as a light source. As an application range of such an electrophotographic technology expands, requirements for the photoreceptor are becoming sophisticated and wide.
As a photoreceptor, conventionally, inorganic photoreceptors comprising a photosensitive layer containing an inorganic photoconductive material such as selenium, zinc oxide or cadmium sulfide as a principal component are widely used.
The inorganic photoreceptor has a basic characteristic as a photoreceptor to some extent, but it has disadvantages that it is difficult to form a photosensitive layer, and plasticity is low and production cost is high. Furthermore, the inorganic photoconductive material is generally highly toxic and there is a large constraint to produce and handle the material.
As described above, since the inorganic photoconductive material and the inorganic photoreceptor using the inorganic photoconductive material have many drawbacks, research and development of an organic photoconductive material is being advanced.
In recent years, the organic photoconductive material is widely researched and developed, and not only it is applied to an electrostatic recording device such as a photoreceptor, but also it is beginning to be applied to a sensor element, an organic electroluminescent (EL) device and the like.
An organic photoreceptor using the organic photoconductive material has advantages that a film forming property of the photosensitive layer is good, plasticity is high, and the photoreceptor is lightweight and highly transparent, and a photoreceptor, which exhibits good sensitivity for a wide-range wavelength region by an appropriate sensitizing method, can be easily designed, and therefore its development is becoming the mainstream.
The organic photoreceptor originally had defects in sensitivity and durability, but these defects are outstandingly improved by development of a function separated type photoreceptor in which a charge generation function and a charge transport function are separated and different substances assume these functions separately. Furthermore, this layered photoreceptor also has advantages that a scope of selection of materials composing the photosensitive layer is wide and a photoreceptor having an arbitrary characteristic can be relatively easily prepared in addition to the aforementioned advantages which the organic photoreceptor has.
As a constitution of such an organic photoreceptor, there can be mentioned various constitutions such as a single layer structure formed by dispersing both a charge generating material and a charge transporting material (also referred to as a “charge transfer substance”) in a binder resin on a substrate, a layered structure in which a charge generation layer formed by dispersing a charge generating material in a binder resin and a charge transporting layer formed by dispersing a charge transporting material in a binder resin are formed on a substrate in this order or in an inverse order, and an inversely layered structure of two layers. Among photoreceptors of these structures, a layered photoreceptor formed by stacking the charge transporting layer on the charge generation layer as a photosensitive layer is widely put to practical use since it has an excellent electrophotographic characteristic and high durability and it enables to design various photoreceptor characteristics because of a high degree of flexibility in material selection.
As a charge generating material used in these function separated type photoreceptors, a variety of substances such as a phthalocyanine pigment, a squarylium dye, an azo pigment, a perylene pigment, a polycyclic quinone pigment, a cyanin dye, a squaric acid dye and a pyrylium salt dye are investigated, and various materials having high lightfastness and a high charge generation capability are proposed.
Further, as the charge transporting material, various compounds such as a pyrazoline compound, a hydrazone compound, a triphenylamine compound, a stilbene compound and an enamine compound are known.
In such a photoreceptor having constitutions proposed or studied as above, various properties such as speeding up, durability and stability of sensitivity is required. Specifically, in response to recent electrophotographic apparatuses of a reversal development system such as digital copying machines and laser printers, it is required to achieve compatibility between the higher sensitivity responding to speeding up as a photoreceptor characteristic and the increase in durability (=longer life) by an improvement of wear resistance and stability of sensitivity. In addition to these, the photoreceptor to be used in a laser printer requires higher image reliability or repetition stability.
However, it is said that these photoreceptors generally have lower durability than inorganic photoreceptors as a large defect. The durability is broadly divided into durability in an aspect of physical properties of electrophotography such as sensitivity, a residual potential, a charging capability and image blurring, and mechanical durability against abrasion or flaw of the photoreceptor surface due to scrubbing. It is known that the primary cause of reduction in durability in an aspect of physical properties of electrophotography is ozone or NOx (nitrogen oxide) generated due to corona discharge or degradation of a charge transporting material contained in a surface layer of the photoreceptor due to light irradiation. Many charge transporting materials made of various structures proposed in large numbers are being improved in terms of durability, but it is not adequate from a practical viewpoint.
Further, a photoreceptor is repeatedly used in a system, and in such a situation, electrophotographic characteristics which are always constant and stable are required. As for such stability and durability, an adequate photoreceptor is not yet attained in any constituent.
That is, with repeated use, problems such as a reduction in potential, an increase in residual potential, and a change in sensitivity arise, the copy quality is deteriorated, and the photoreceptor becomes inoperative. The causes of these degradation is not fully explained, but some factors are conceivable.
For example, it is known that ozone emitted from a corona discharge charger, and oxidizing gases such as nitrogen oxide cause significant damages to the photosensitive layer. These oxidizing gases chemically change materials in the photosensitive layer to cause various changes of characteristics. For example, oxidizing gases cause reduction in a charging potential, an increase in a residual potential, and deterioration of a resolution power due to a decrease in surface resistance, and consequently image blurring such as white spots or black stripes is generated on an output image to deteriorate the image quality seriously and shorten the life of the photoreceptor. Against such phenomena, a proposal of taking countermeasures in which the gas around the corona charger is efficiently exhausted or replaced to avoid a direct effect of the gas on the photoreceptor, and a proposal of adding an antioxidant or a stabilizer to the photosensitive layer to prevent degradation are presented.
For example, in Japanese Unexamined Patent Publication No. 62-105151, it is disclosed to add an antioxidant having a triazine ring and a hindered phenol skeleton in a molecule to a photosensitive layer, and in Japanese Unexamined Patent Publication No. Sho 63 (1988)-18355, it is disclosed to add a specific hindered amine to a photosensitive layer. Further, in Japanese Unexamined Patent Publication No. Sho 63 (1988)-4238, Japanese Unexamined Patent Publication No. Sho 63 (1988)-216055 and Japanese Unexamined Patent Publication No. Hei 3 (1991)-172852, it is disclosed to add trialkylamine and aromatic amine to a photosensitive layer, and in Japanese Unexamined Patent Publication No. Hei 5 (1993)-158258, it is disclosed to add amine dimer to a photosensitive layer, but these methods are still inadequate.
That is, an adequate effect of ozone resistance is not yet achieved by such conventional techniques, and the current state of affairs is that a practically adverse effect that addition of such an antioxidant causes the deterioration of electrophotographic characteristics such as sensitivity and a residual potential still remains. Accordingly, a proposal of a novel material which improves ozone resistance and does not have an adverse effect on the electrophotographic characteristics at all is desired.